The present invention relates to a method of producing a semiconductor device such as a power semiconductor module comprising a power semiconductor.
Power semiconductor modules capable of working even under large current and high voltage have recently been used in various fields. Such power modules comprise a power semiconductor such as an Insulated Gate Bipolar Transistor (IGBT) and a Free Wheeling Diode (FWD) as a main component.
FIG. 11 is a schematic cross-sectional view showing a principal part of a conventional power semiconductor module.
In a power semiconductor module 100 shown in FIG. 11, a semiconductor chip 103 of a power semiconductor, etc. is connected by a solder layer 102 onto an insulating substrate 101, which comprises a ceramic board 101a of aluminum nitride (AlN), etc. and conductor layers 101b and 101c of copper (Cu), aluminum (Al), etc. formed on the both surface thereof. One surface of the insulating substrate 101 is connected to the semiconductor chip 103 in this manner, and the opposite surface is connected by a solder layer 104 to a radiating base 105 composed of a metal such as copper to dissipate heat generated in the semiconductor chip 103.
However, in the production of the power semiconductor module 100 having such a structure, two members with different heat expansion coefficients, the insulating substrate 101 having the ceramic board 101a and the metal radiating base 105, are connected by the solder layer 104, whereby the originally flat radiating base 105 is warped after the soldering occasionally.
FIG. 12 is a schematic cross-sectional view showing a principal part of the warped radiating base. It should be noted that the same reference signs are used in FIGS. 11 and 12 for representing common components.
For example, in the case of using aluminum nitride for the ceramic board 101a of the insulating substrate 101 and using copper for the radiating base 105, aluminum nitride has a heat expansion coefficient of about 4.5 ppm/K and copper has a heat expansion coefficient of about 16.5 ppm/K, resulting in a relatively large difference in the coefficients. Thus, copper is shrunk more than aluminum nitride in a cooling step after the soldering, whereby the radiating base 105 is often convexly warped in the direction of the insulating substrate 101. When the radiating base 105 is warped in this manner, an assembling step after the soldering, etc. is adversely affected or the performance of the power semiconductor module 100 is occasionally deteriorated depending on the warpage.
Some proposals have been made to prevent convex warpage of radiating members such as the radiating base 105, formed in a connecting process of soldering, etc. The proposals include a module obtained by connecting a radiating metal layer to a ceramic board and by connecting the radiating metal layer to a radiating member via a brazing material layer with a connection area of 300 mm2 or less (JP-A-2004-140199). The proposals further include a method of connecting a ceramic board to a radiating member after convexly warping the radiating member to the side opposite to the ceramic board, thereby compensating for warpage formed in the connecting process (JP-A-2003-46032 and JP-A-4-96355).
Additionally, in order to decrease air bubbles in the soldering layer, the proposals further include a method of connecting a ceramic board to a radiating member after attaching copper plates having different thicknesses to both surfaces of the ceramic board, the copper plate on the side of the radiating member having a larger thickness, thereby convexly warping the ceramic board to the radiating member in the connecting process by utilizing the thickness difference, to remove air bubbles in the soldering layer (JP-A-10-270612).
However, in production of the above power semiconductor modules, in the case of using a solder for connecting members, particularly members with different heat expansion coefficients, the following problems can result.
Currently many solders for connecting members of electronic devices and parts including the power semiconductor modules contain lead (Pb). When electronic devices and parts using lead-containing solders are discarded, left in an outdoor location, and exposed to acid rain, etc., lead in the solders may be eluted off to cause environment contamination. Therefore, it is preferred that so-called lead-free solders, which are mainly composed of tin (Sn) etc. without lead, are used in various electronic devices and parts.
The lead-free solders have higher hardness as compared with the lead-containing solders. In the case of using a lead-containing solder for connecting the insulating substrate 101 and the flat radiating base 105 of the power semiconductor module 100 shown in FIGS. 11 and 12, though the radiating base 105 may be convexly warped by sintering in the direction of the insulating substrate 101, the solder layer 104 can be creep-deformed immediately after the soldering because of the softness of the solder, to relax the stress between them. As a result, the warpage of the radiating base 105 is removed, and the radiating base 105 is returned to the original flat or approximately flat state.
In contrast, in the case of using a lead-free solder for the connection, the solder is hard and thus the solder layer 104 is not creep-deformed, so that the radiating base 105 is not returned to the original flat state with the convex warpage remaining. The amount of the warpage is large, approximately 200 to 500 μm, and as a result an assembling step after the soldering is adversely affected or the performance of the power semiconductor module 100 is deteriorated occasionally as described above.
FIG. 13 is a schematic cross-sectional view showing a principal part of the step of assembling the power semiconductor module. It should be noted that the same reference signs are used in FIGS. 11, 12, and 13 for representing common components.
As shown in FIG. 13, in the power semiconductor module 100, generally, the insulating substrate 101 and the radiating base 105 are solder-connected, and then the radiating base 105 is fixed to a cooling fin 200 by a screw, etc.
In the case of using a lead-containing solder for connecting the insulating substrate 101 and the radiating base 105, the convex warpage of the radiating base 105, formed in the soldering step, is removed thereafter. Therefore, the contact thermal resistance between the radiating base 105 and the cooling fin 200 is relatively small, and heat generated in the semiconductor chip 103 is efficiently dissipated from the radiating base 105.
In contrast, in a case where a lead-free solder is used for connecting to the insulating substrate 101 and thus the radiating base 105 is largely convex-warped in the direction of the insulating substrate 101, a large gap 201 is formed between the radiating base 105 and the flat surface of the cooling fin 200 as shown in FIG. 13. When such a gap 201 is formed, the contact thermal resistance is increased, and the efficiency of dissipating heat generated in the semiconductor chip 103 is lowered, so that the temperature of the connection part of the semiconductor chip 103 may be abnormally increased to cause thermal destruction. Further, in a case where the radiating base 105 is largely convex-warped toward the insulating substrate 101, occasionally a problem occurs, for example, the ceramic board 101a is cracked in the step of screwing the radiating base 105 to the cooling fin 200.
The warpage can be reduced by using materials having low heat expansion coefficients such as copper molybdenum (CuMo) composite materials and aluminum silicon carbides (AlSiC) for the radiating base 105 instead of copper to reduce the heat expansion coefficient difference between the radiating base 105 and the insulating substrate 101. However, as compared with copper, these materials are poorer in thermal conductivity and disadvantageous in heat dissipation though they have low heat expansion coefficients. Further, these materials are more costly than copper.